Conflict with my boss

Hi all,

I really need help creating a letter for my upper management/HR.. Im having conflict with my boss and I had enough!!... I'm scared to quit Im afraid I might not find a job quick enough I'm a single mom and can't aford to stay without a job...

I work from Sunday/Thrusday I've been working at this job for a little less then a year.. I had plans to take my two kids camping for 4th of July weekend and now I can't go.. I've asked for the Sunday off a long time ago and now my boss changed her mind and she doesn't want to give me the day off.. So I'll have to canceled all of my kids plans cause I'll have to attend to work.. This is just one sample of the thing my boss does to me.. I'm very upset and don't know who to confront this situation. By the way, This will be my first time asking for the day off.. Im always at work and on time. Always complete all my task.

Please help me create a letter to HR explaning my issue.

Thanks for all your help!